Foam control compositions which are designed for use in aqueous systems may or may not contain water, but in both cases, they are, preferably, water dispersible. Their preparation and properties are well known and are already disclosed in the patent and scholarly literature. For example, DE 2626994, GB197254, DE 3013923, U.S. Pat. No. 5,543,082, JP 58007335 and the review articles listed below report the preparation of such formulations, their use as foam control additives in the textiles, food, agriculture and paper industries, and investigate their mechanism of action and deactivation.
R. S. Bhute, J. Scient. Ind. Res. 1971, vol 30, 241 PA0 K. Roberts et al, Journal of Colloid and Interface Science, 1977, vol 62, 264 PA0 S. Ross, Chemical Engineering Process, 1967, vol 63, 41 PA0 The emulsion viscosity should be low for ease of use PA0 The emulsion should be stable to temperatures as high as 50.degree. C. PA0 The emulsion once diluted with water, should be free of unemulsified oil and non-dispersed solid particles.
Silicone foam control formulations have been known in the art for many years and are widely recognised as being the most effective agents in a plurality of situations. They are formulated to be active in either aqueous or non-aqueous systems. In the case of the aqueous systems, the silicone foam control agent can be either an emulsion (water in oil, oil in water, or multiple phase) or a water dispersible composition. Silicone foam control agents in the form of an emulsion are widely used today.
The first step in the preparation of a silicone antifoam emulsion is the preparation of the silicone antifoam compound. This is typically a mixture of a silicone fluid and fine solid particles, frequently silica, that has undergone a heat treatment at 100-300.degree. C. for several hours in the presence of catalysts, such as alkali silanolate or alkali hydroxide to increase effectiveness, as described, for example in U.S. Pat. Nos. 3,235,509 and 3,560,401. Often, the antifoam compound also contains non-silicone oils (EP 687725, WO 95 05,580, CN 1 088 128) to improve performance or compatibility in certain applications.
Antifoam compounds such as these described above can not be easily used as such in aqueous systems because they do not disperse easily in water. This not only reduces their potency but it may cause other undesirable effects, such as, silicone spotting of fabrics in textiles. It is thus indispensable that they be made water compatible.
Water compatibility can be achieved by either blending surfactants in with the antifoam compound (U.S. Pat. Nos. 3,984,200, 4,076,648, JP 05261206) to produce self-emulsifiable compounds, or by emulsifying the antifoam compound to an oil in water or multiple phase emulsion. While there exists a large patent literature on self-emulsifiable compounds, the majority of material commercially used is of the emulsion type. The main reason is ease of use: the antifoam emulsion can be easily and uniformly dispersed into an aqueous phase using only slight or moderate mixing.
The preparation of silicone antifoam emulsions, while appearing easy, is in fact complicated due to the numerous variables, demonstrated empirically, that affect performance. These can arise from changes in the composition of the antifoam compound, in the composition and nature of the emulsion, and in the nature of the foaming system.
The antifoam emulsion droplet size is one of the most important factors controlling the antifoam activity in a given foaming medium. Antifoam effectiveness however, is not the only requirement. In particular, in sensitive industries such as textiles, the antifoam emulsion quality is also of importance:
In addition, textile formulators require that the emulsion, when diluted with water, form stable dilutions that they can then sell at a profit. This is a particularly difficult prerequisite. Effective antifoam emulsions use thickeners in amounts optimised for the undiluted emulsion to prevent droplet creaming and coalescing. Upon dilution with water, the thickener levels are drastically reduced and are thus no longer capable of imparting the required stability: the diluted emulsion separates. The separation may not be obvious, as, in certain cases, there is no phase separation visible to the naked eye. However, if the diluted emulsion be tested for antifoam efficiency, it will be found that either the top or the bottom layer (depending on whether the droplets are lighter or heavier than the continuous phase) have retained efficiency. The rest of the dilution, being depleted of antifoam droplets, exhibits drastically reduced foam control.
A way to overcome this would be to use higher quantities of thickeners but this would result in an unacceptably high emulsion viscosity. Alternatively, the emulsion droplet could be made smaller through processing (eg, homogenising) to achieve the required stability, but then the effectiveness of the antifoam would be reduced.
European Patent No 0 769 548 teaches the preparation of silicone antifoam emulsions using alkylpolyglucosides and anionic surfactants as the emulsifiers and xanthan gum as the thickener. The antifoam compound is based on a blend of silicone oil and hydrophobic silica. The finished formulation is stable for 180 days at 40.degree. C. and exhibits foam control.
German Patent No 42 37 754 teaches the use of linear polyurethane dispersions or emulsions as protective colloids in the preparation of silicone antifoam emulsions. The emulsifiers are non-ionic polyethylene oxide modified triglycerides or alcohol ethoxylates. The antifoam compound is based on a silicone oil and hydrophobic silica. The finished formulation exhibits foam control and stability in a variety of latexes.
German Patent No 30 13 923 teaches the use of water soluble molecules having the general formula R.sup.1 {CH.sub.2 CH(R.sup.2)O}R.sup.3 where R.sup.1 is either an alkyl or alkoxy group, R.sup.2 is either hydrogen or methyl and R.sup.3 is an alkyl group for the preparation of silicone antifoam emulsions. These compounds help in the emulsification and improve the dispersibility and foam control efficiency of the emulsion. The emulsifiers are non-ionic triglycerides and alcohol ethoxylates. The antifoam compound is based on a blend of a silicone oil and hydrophobic silica. No thickeners or protective colloids are disclosed in this patent. The finished emulsion has low viscosity and exhibits foam control.
German Patent No 43 43 185 teaches the use of polyalkylene oxide modified siloxanes as emulsifiers for silicone antifoam compounds that are based on blends of polyalkylene oxide modified siloxanes and hydrophobic silica. The siloxane chain is either linear or branched and the polyalkylene oxide moiety is either pendant or end-blocked. No protective colloids or thickeners are disclosed in this patent. The final emulsion showed compatibility and foam control in acrylic latexes.
The limitation of the formulations described in the patents above is that they do not embody all the requirements that an emulsion destined for the textiles market demands.